International Collaboration in Chemistry: Mechanism for Cation Exchange and Doping in Semiconducting Nanostructures

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Phillip Geissler from the University of California Berkeley is supported in an International Collaboration in Chemistry award by the Macromolecular, Supramolecular and Nanochemistry program and the Chemical Theory, Models and Computational Methods program; his collaborator, Eran Rabani at Tel Aviv University, is simultaneously supported by the U.S.-Israel Binational Science Foundation. The project is motivated by the fact that the past 20 years have seen a revolution in chemists' ability to make materials on extremely small scales, notably in the form of "nanocrystals" (nanometer-sized excerpts of crystalline materials). Realizing the immense technological promise of nanocrystals, however, requires much more control over their composition, shape, structure, and relative arrangements than is currently possible. This project concerns a recently discovered route towards gaining control of this kind: exchanging one set of ions in a nanocrystal with a chemically distinct set of ions, remarkably without altering the nanocrystal's structure. The mechanism, energetics, dynamics, and physical consequences of such "cation exchange" are very poorly understood, greatly limiting its precise application. The project uses tools of theory and computation to develop and explore models that shed light on the chemical underpinnings of ion exchange. It further evaluates the consequences of exchange on electronic properties relevant to desirable technological advances. Specific activities of this project include (i) construction of models for exploring reaction mechanisms of doping and ion exchange, (ii) development and application of state-of-the-art computational techniques needed to explore the long-time behaviors of these models, and (iii) the assessment of resulting electronic properties. Interaction models are built through judicious comparison with electronic structure calculations and experimental measurements. Molecular dynamics simulations combined with techniques such as transition path sampling are employed to examine how impurities/cations enter the nanocrystal from solution, how they diffuse through the host lattice, and how they exchange places with host atoms. In order to calculate emergent electronic properties, semiempirical pseudopotential models adequate for doped materials and heterostructures are developed. Finally, a theory for heavily doped nanocrystals in the spirit of a "hydrogenic-like" model for multiple impurities is being constructed to elucidate at a fundamental level the alignment of bands, localization of excitons, electron-hole interactions, and how these properties depend on the size, shape, and choice of materials.